System and method for assessing coupling between an electrode and tissue

ABSTRACT

A system and method for assessing a degree of coupling between an electrode and tissue in a body is provided. Values for first and second components of a complex impedance (e.g., resistance and reactance or impedance magnitude and phase angle) between the electrode and the tissue are obtained. These values are used together with a standardization value indicative of a deviation from a reference standard by a parameter associated with at least one of the body, the electrode and another component of the system to calculate a coupling index that is indicative of a degree of coupling between the electrode and the tissue. The coupling index may be displayed to a clinician in a variety of ways to indicate the degree of coupling to the clinician. The system and method find particular application in ablation of tissue by permitting a clinician to create lesions in the tissue more effectively and safely.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/650,060, filed 30 Dec. 2009 (the '060 application), now pending,which is a continuation-in-part of U.S. patent application Ser. No.12/253,637 filed Oct. 17, 2008 (the '637 application), which is acontinuation-in-part of U.S. patent application Ser. No. 12/095,688filed May 30, 2008 (the '688 application), which is a national stageapplication of International application no. PCT/US2006/061714 filedDec. 6, 2006 (the '714 application) which claims the benefit of U.S.Provisional patent application No. 60/748,234 filed Dec. 6, 2005 (the'234 application). The '060 application, the '637 application, the '688application, the '714 application and the '234 application are allhereby incorporated by reference as though fully set forth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates to a system and method for assessing the degreeof coupling between an electrode and tissue in a body. In particular,the instant invention relates to a system and method for assessing thedegree of electrical coupling between electrodes on an diagnostic and/ortherapeutic medical device such as a mapping or ablation catheter andtissue, such as cardiac tissue.

b. Background Art

Electrodes are used on a variety of diagnostic and/or therapeuticmedical devices. For example, electrodes may be used on cardiac mappingcatheters to generate an image of the internal geometry of a heart andelectrical potentials within the tissue. Electrodes are also used onablation catheters to create tissue necrosis in cardiac tissue tocorrect conditions such as atrial arrhythmia (including, but not limitedto, ectopic atrial tachycardia, atrial fibrillation, and atrialflutter). Arrhythmia can create a variety of dangerous conditionsincluding irregular heart rates, loss of synchronous atrioventricularcontractions and stasis of blood flow which can lead to a variety ofailments and even death. It is believed that the primary cause of atrialarrhythmia is stray electrical signals within the left or right atriumof the heart. The ablation catheter imparts ablative energy (e.g.,radiofrequency energy, cryoablation, lasers, chemicals, high-intensityfocused ultrasound, etc.) to cardiac tissue to create a lesion in thecardiac tissue. This lesion disrupts undesirable electrical pathways andthereby limits or prevents stray electrical signals that lead toarrhythmias.

The safety and effectiveness of many of diagnostic and/or therapeuticdevices is often determined in part by the proximity of the device andthe electrodes to the target tissue. In mapping catheters, the distancebetween the electrodes and the target tissue affects the strength of theelectrical signal and the identity of the mapping location. The safetyand effectiveness of ablation lesions is determined in part by theproximity of the ablation electrode to target tissue and the effectiveapplication of energy to that tissue. If the electrode is too far fromthe tissue or has insufficient contact with the tissue, the lesionscreated may not be effective. On the other hand, if the catheter tipcontaining the electrode contacts the tissue with excessive force, thecatheter tip may perforate or otherwise damage the tissue (e.g., byoverheating). It is therefore beneficial to assess the quality ofcontact between the electrode and the tissue.

Contact between a catheter electrode and tissue has typically beendetermined using one or more of the following methods: clinician sense,fluoroscopic imaging, intracardiac echo (ICE), atrial electrograms(typically bipolar D-2), pacing thresholds, evaluation of lesion size atnecropsy and measurement of temperature change at the energy deliverysite. Each of these methods has disadvantages, however.

Although a clinician can evaluate contact based on tactile feedback fromthe catheter and prior experience, the determination depends largely onthe experience of the clinician and is also subject to change based onvariations in the mechanical properties of catheters used by theclinician. The determination is particularly difficult when usingcatheters that are relatively long (such as those used to enter the leftatria of the heart).

Because fluoroscopic images are two-dimensional projections and bloodand myocardium attenuate x-rays similarly, it is difficult to quantifythe degree of contact and to detect when the catheter tip is not incontact with the tissue. Fluoroscopic imaging also exposes the patientand clinician to radiation.

Intracardiac echo is time consuming and it is also difficult to alignthe echo beam with the ablation catheter. Further, intracardiac echodoes not always permit the clinician to confidently assess the degree ofcontact and can generate unacceptable levels of false positives andfalse negatives in assessing whether the electrode is in contact withtissue.

Atrial electrograms do not always correlate well to tissue contact andare also prone to false negatives and positives. Pacing thresholds alsodo not always correlate well with tissue contact and pacing thresholdsare time-consuming and also prone to false positives and false negativesbecause tissue excitability may vary in hearts with arrhythmia.Evaluating lesion size at necropsy is seldom available in humansubjects, provides limited information (few data points) and, further,it is often difficult to evaluate the depth and volume of lesions in theleft and right atria. Finally, temperature measurements provide limitedinformation (few data points) and are difficult to evaluate in the caseof irrigated catheters.

A more recent method of assessing contact between the catheter electrodeand tissue is the use of force sensors incorporated into the catheter tomeasure contact force between the catheter tip and tissue. Contactforce, however, does not directly measure how well electrical energy iscoupled between the catheter electrode and tissue. Particularly forradio-frequency (RF) ablation catheters, a measure of electricalcoupling may be more relevant to ablation safety and efficacy indifferent types of tissue and in different types of catheter tip totissue surface alignment (e.g., perpendicular versus parallelorientation). The use of force sensors also requires significantstructural adjustments and technological advances for use inconventional ablation catheters.

Contact between the catheter electrode and tissue has also beenevaluated by measuring impedance between the catheter electrode and anelectrode disposed on the a patient's skin. During radio frequency (RF)ablation, the generation of RF energy is controlled by an ablationgenerator. The ablation generator displays a measure of the magnitude ofimpedance (Z). This measurement, however, does not correlate well withthe more localized contact between the catheter electrode and tissuebecause it measures the impedance provided not just by the local targettissue, but the entire impedance from the electrode to a cutaneousreturn electrode through various body tissues and fluids. Furthermore,generator reported impedance is usually infrequently obtained and at lowresolution (about 1Ω). It is also not readily available to the clinicianin a format that allows easy interpretation and correlation to tissuecontact.

In priority U.S. patent application Ser. No. 12/253,637, a system andmethod are provided for determining a degree of coupling between acatheter electrode and tissue in which an electrical coupling index(ECI) is generated as an indicator of impedance at the interface of theelectrode and the target tissue. Because the coupling index isindicative of impedance at the interface of the electrode and targettissue, the inventive system and method provide a better assessment ofcoupling between the electrode and the tissue and the index permits abetter assessment of the degree of coupling between an electrode andtissue as compared to prior art systems. The index, however, is subjectto variability based on changes to properties associated with patientbodies (e.g. differences in body temperature among patients) andcomponents of the system (e.g., differences resulting from the use ofdifferent ablation catheters).

The inventors herein have recognized a need for a system and method fordetermining a degree of coupling between a catheter electrode and tissuethat will minimize and/or eliminate one or more of the above-identifieddeficiencies.

BRIEF SUMMARY OF THE INVENTION

It is desirable to provide a system and method for determining thedegree of coupling between an electrode and a tissue in a body. Inparticular, it is desirable to be able to determine a degree ofelectrical coupling between electrodes on an diagnostic and/ortherapeutic medical device such as a mapping or ablation catheter andtissue, such as cardiac tissue.

A system for assessing a degree of coupling between an electrode and atissue in a body in accordance with one embodiment of the presentinvention includes an electronic control unit configured to acquirevalues for first and second components of a complex impedance betweenthe electrode and the tissue. The first and second components maycomprise, for example, a resistance between the electrode and the tissueand a reactance between the electrode and the tissue. Alternatively, thefirst and second components may comprise an impedance magnitude betweenthe electrode and the tissue and an impedance phase angle. Theelectronic control unit is further configured to calculate a couplingindex responsive to the values and a standardization value indicative ofa deviation from a reference standard by a parameter associated with atleast one of the body, the electrode and another component of thesystem. The standardization value may, for example, be indicative of adifference in body temperature in a patient relative to a reference orstandard temperature or a difference in the size of the electroderelative to a reference or standard electrode size. The coupling indexis indicative of a degree of coupling between the electrode and thetissue. In accordance with one aspect of the invention, the couplingindex may be displayed to a clinician on a display device in a varietyof ways to provide to permit easy interpretation and correlation of themeasured impedance to coupling between the electrode and tissue.

Similarly, an article of manufacture for assessing a degree of couplingbetween an electrode and a tissue in a body in accordance with anotherembodiment of the present invention includes a computer storage mediumhaving a computer program encoded thereon for determining a degree ofcoupling between an electrode and tissue in a body. The computer programincludes code for calculating a coupling index responsive to values forfirst and second components of a complex impedance between the electrodeand the tissue and a standardization value indicative of a deviationfrom a reference standard by a parameter associated with at least one ofthe body, the electrode and another component of a system controllingthe electrode. The coupling index is indicative of a degree of couplingbetween the electrode and the tissue.

A method for assessing a degree of coupling between an electrode and atissue in a body in accordance with another embodiment of the presentinvention includes the step of acquiring values for first and secondcomponents of a complex impedance between the electrode and the tissue.Again, the first and second components may comprise, for example, aresistance between the electrode and the tissue and a reactance betweenthe electrode and the tissue. Alternatively, the first and secondcomponents may comprise an impedance magnitude between the electrode andthe tissue and an impedance phase angle. The method further includes thestep of calculating a coupling index responsive to the values and astandardization value indicative of a deviation from a referencestandard by a parameter associated with at least one of the body, theelectrode and another component of a system controlling the electrode.The coupling index is indicative of a degree of coupling between theelectrode and the tissue.

The above-described system, article of manufacture and method areadvantageous because they provide a true measure of impedance at theinterface of the electrode and the target tissue and, therefore, providea better assessment of coupling between the electrode and the tissue.Moreover, the measurement is standardized such that it will provide asimilar measure despite variation in parameters associated with thebody, the electrode, and the system as a whole. Further, the system,article of manufacture and method provide an indicator of coupling(i.e., the coupling index) in a format that allows easy interpretationand correlation to tissue contact by the clinician.

The coupling index can be used in a wide variety of diagnostic andtherapeutic devices. In ablation catheters, for example, procedures canbe conducted more efficiently and with higher success rates and fewercomplications because the coupling index provides an indication of theenergy delivered to the target tissue and the proximity of the catheterand electrode to the target tissue and may also enable a clinician toidentify the orientation or angle of the ablation electrode relative tothe tissue. The index also can act as a proximity sensor indicating thedistance between a diagnostic or therapeutic device and a target tissue.For example, in a transseptal access sheath, the coupling index providesan indication that the sheath is approaching (and potentially slippingaway from) the septum. In a mapping catheter, the coupling index mayimprove geometric modeling of the tissue surface by, for example,identifying the most relevant electrode readings and/or allowingmultiple electrodes to simultaneously obtain relevant measurements.

The foregoing and other aspects, features, details, utilities andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic view of a system in accordance with the presentteachings.

FIG. 2 is a simplified schematic diagram illustrating how impedance isdetermined in accordance with the present teachings.

FIG. 3 is a diagrammatic and block diagram illustrating the approach inFIG. 2 in greater detail.

FIG. 4 is a series of diagrams illustrating complex impedance variationsduring atrial tissue ablation and cardiac tissue contact over thirty(30) seconds.

FIG. 5 is s series of diagrams illustrating variations in a couplingindex during atrial tissue ablation and cardiac tissue contact over onehundred and sixty (160) seconds.

FIG. 6 is a screen display illustrating possible formats for presentinga coupling index to a clinician.

FIG. 7 is a diagrammatic view of a multi-electrode, array catheterillustrating one embodiment of a system in accordance with presentteachings.

FIG. 8 is a flow chart illustrating one embodiment of a method inaccordance with the present teachings.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 1illustrates one embodiment of a system 10 for one or more diagnostic andtherapeutic functions including components providing an improvedassessment of a degree of coupling between an electrode 12 on a catheter14 and a tissue 16 in a body 17. In the illustrated embodiment, tissue16 comprises heart or cardiac tissue. It should be understood, however,that the present invention may be used to evaluate coupling betweenelectrodes and a variety of body tissues. Further, although electrode 12is illustrated as part of a catheter 14, it should be understood thatthe present invention may be used to assess a degree of coupling betweenany type of electrode and tissue including, for example, intracardiacelectrodes, needle electrodes, patch electrodes, wet brush electrodes(such as the electrodes disclosed in commonly assigned U.S. patentapplication Ser. No. 11/190,724 filed Jul. 27, 2005, the entiredisclosure of which is incorporated herein by reference) and virtualelectrodes (e.g., those formed from a conductive fluid medium such assaline including those disclosed in commonly assigned U.S. Pat. No.7,326,208 issued Feb. 5, 2008, the entire disclosure of which isincorporated herein by reference). In addition to catheter 14, system 10may include patch electrodes 18, 20, 22, an ablation generator 24, atissue sensing circuit 26, an electrophysiology (EP) monitor 28 and asystem 30 for visualization, mapping and navigation of internal bodystructures which may include an electronic control unit 32 in accordancewith the present invention and a display device 34 among othercomponents.

Catheter 14 is provided for examination, diagnosis and treatment ofinternal body tissues such as tissue 16. In accordance with oneembodiment of the invention, catheter 14 comprises an ablation catheterand, more particularly, an irrigated radio-frequency (RF) ablationcatheter. It should be understood, however, that the present inventioncan be implemented and practiced regardless of the type of ablationenergy provided (e.g., cryoablation, ultrasound, etc.) Catheter 14 isconnected to a fluid source 36 having a biocompatible fluid such assaline through a pump 38 (which may comprise, for example, a fixed rateroller pump or variable volume syringe pump with a gravity feed supplyfrom fluid source 36 as shown) for irrigation. Catheter 14 is alsoelectrically connected to ablation generator 24 for delivery of RFenergy. Catheter 14 may include a cable connector or interface 40, ahandle 42, a shaft 44 having a proximal end 46 and a distal 48 end (asused herein, “proximal” refers to a direction toward the end of thecatheter near the clinician, and “distal” refers to a direction awayfrom the clinician and (generally) inside the body of a patient) and oneor more electrodes 12, 50, 52. Catheter 14 may also include otherconventional components not illustrated herein such as a temperaturesensor, additional electrodes, and corresponding conductors or leads.

Connector 40 provides mechanical, fluid and electrical connection(s) forcables 54, 56 extending from pump 38 and ablation generator 24.Connector 40 is conventional in the art and is disposed at a proximalend of catheter 14.

Handle 42 provides a location for the clinician to hold catheter 14 andmay further provides means for steering or guiding shaft 44 within body17. For example, handle 42 may include means to change the length of aguidewire extending through catheter 14 to distal end 48 of shaft 44 tosteer shaft 44. Handle 42 is also conventional in the art and it will beunderstood that the construction of handle 42 may vary.

Shaft 44 is an elongated, tubular, flexible member configured formovement within body 17. Shaft 44 support electrodes 12, 50, 52associated conductors, and possibly additional electronics used forsignal processing or conditioning. Shaft 44 may also permit transport,delivery and/or removal of fluids (including irrigation fluids andbodily fluids), medicines, and/or surgical tools or instruments. Shaft44 may be made from conventional materials such as polyurethane anddefines one or more lumens configured to house and/or transportelectrical conductors, fluids or surgical tools. Shaft 44 may beintroduced into a blood vessel or other structure within body 17 througha conventional introducer. Shaft 44 may then be steered or guidedthrough body 17 to a desired location such as tissue 16 with guide wiresor other means known in the art.

Electrodes 12, 50, 52 are provided for a variety of diagnostic andtherapeutic purposes including, for example, electrophysiologicalstudies, catheter identification and location, pacing, cardiac mappingand ablation. In the illustrated embodiment, catheter includes anablation tip electrode 12 at distal end 48 of shaft 44 and a pair ofring electrodes 50, 52. It should be understood, however, that thenumber, orientation and purpose of electrodes 12, 50, 52 may vary.

Patch electrodes 18, 20, 22 provide RF or navigational signal injectionpaths and/or are used to sense electrical potentials. Electrodes 18, 20,22 may also have additional purposes such as the generation of anelectromechanical map. Electrodes 18, 20, 22 are made from flexible,electrically conductive material and are configured for affixation tobody 17 such that electrodes 18, 20, 22 are in electrical contact withthe patient's skin. Electrode 18 may function as an RFindifferent/dispersive return for the RF ablation signal. Electrodes 20,22 may function as returns for the RF ablation signal source and/or anexcitation signal generated by tissue sensing circuit 26 as described ingreater detail hereinbelow. In accordance with one aspect of the presentinvention discussed hereinbelow, electrodes 20, 22 are preferably spacedrelatively far apart. In the illustrated embodiment, electrodes 20, 22,are located on the medial aspect of the left leg and the dorsal aspectof the neck. Electrodes 20, 22, may alternatively be located on thefront and back of the torso or in other conventional orientations.

Ablation generator 24 generates, delivers and controls RF energy used byablation catheter 14. Generator 24 is conventional in the art and maycomprise the commercially available unit sold under the model numberIBI-1500T RF Cardiac Ablation Generator, available from IrvineBiomedical, Inc. Generator 24 includes an RF ablation signal source 54configured to generate an ablation signal that is output across a pairof source connectors: a positive polarity connector SOURCE (+) which mayconnect to tip electrode 12; and a negative polarity connector SOURCE(−) which may be electrically connected by conductors or lead wires toone of patch electrodes 18, 20, 22 (see FIG. 2). It should be understoodthat the term connectors as used herein does not imply a particular typeof physical interface mechanism, but is rather broadly contemplated torepresent one or more electrical nodes. Source 54 is configured togenerate a signal at a predetermined frequency in accordance with one ormore user specified parameters (e.g., power, time, etc.) and under thecontrol of various feedback sensing and control circuitry as is know inthe art. Source 54 may generate a signal, for example, with a frequencyof about 450 kHz or greater. Generator 24 may also monitor variousparameters associated with the ablation procedure including impedance,the temperature at the tip of the catheter, ablation energy and theposition of the catheter and provide feedback to the clinician regardingthese parameters. The impedance measurement output by generator 24,however, reflects the magnitude of impedance not only at tissue 16, butthe entire impedance between tip electrode 12 and the correspondingpatch electrode 18 on the body surface. The impedance output bygenerator 24 is also not easy to interpret and correlate to tissuecontact by the clinician.

Tissue sensing circuit 26 provides a means, such as tissue sensingsignal source 56, for generating an excitation signal used in impedancemeasurements and means, such as complex impedance sensor 58, forresolving the detected impedance into its component parts. Signal source56 is configured to generate an excitation signal across sourceconnectors SOURCE (+) and SOURCE (−) (See FIG. 2). Source 56 may outputa signal having a frequency within a range from about 1 kHz to over 500kHz, more preferably within a range of about 2 kHz to 200 kHz, and evenmore preferably about 20 kHz. In one embodiment, the excitation signalis a constant current signal, preferably in the range of between 20-200μA, and more preferably about 100 μA. As discussed below, the constantcurrent AC excitation signal generated by source 56 is configured todevelop a corresponding AC response voltage signal that is dependent onthe complex impedance of tissue 16 and is sensed by complex impedancesensor 58. Sensor 58 resolves the complex impedance into its componentparts (i.e., the resistance (R) and reactance (X) or the impedancemagnitude (|Z|) and phase angle (∠Z or φ)). Sensor 58 may includeconventional filters (e.g., bandpass filters) to block frequencies thatare not of interest, but permit appropriate frequencies, such as theexcitation frequency, to pass as well as conventional signal processingsoftware used to obtain the component parts of the measured compleximpedance.

It should be understood that variations are contemplated by the presentinvention. For example, the excitation signal may be an AC voltagesignal where the response signal comprises an AC current signal.Nonetheless, a constant current excitation signal is preferred as beingmore practical. It should be appreciated that the excitation signalfrequency is preferably outside of the frequency range of the RFablation signal, which allows the complex impedance sensor 58 to morereadily distinguish the two signals, and facilitates filtering andsubsequent processing of the AC response voltage signal. The excitationsignal frequency is also preferably outside the frequency range ofconventionally expected electrogram (EGM) signals in the frequency rangeof 0.05-1 kHz. Thus, in summary, the excitation signal preferably has afrequency that is preferably above the typical EGM signal frequenciesand below the typical RF ablation signal frequencies.

Circuit 26 is also connected, for a purpose described hereinbelow,across a pair of sense connectors: a positive polarity connector SENSE(+) which may connect to tip electrode 12; and a negative polarityconnector SENSE (−) which may be electrically connected to one of patchelectrodes 18, 20, 22 (see FIG. 2; note, however, that the connectorSENSE (−) should be connected to a different electrode of electrodes 18,20, 22 relative to the connector SOURCE (−) as discussed below). Itshould again be understood that the term connectors as used herein doesnot imply a particular type of physical interface mechanism, but israther broadly contemplated to represent one or more electrical nodes.

Referring now to FIG. 2, connectors SOURCE (+), SOURCE (−), SENSE (+)and SENSE (−) from a three terminal arrangement permitting measurementof the complex impedance at the interface of tip electrode 12 and tissue16. Complex impedance can be expressed in rectangular coordinates as setforth in equation (1):

Z=R+jX  (1)

where R is the resistance component (expressed in ohms); and X is areactance component (also expressed in ohms). Complex impedance can alsobe expressed polar coordinates as set forth in equation (2):

Z=r·e ^(jθ) =|Z|·e ^(j∠Z)  (2)

where |Z| is the magnitude of the complex impedance (expressed in ohms)and ∠Z=θ is the phase angle expressed in radians. Alternatively, thephase angle may be expressed in terms of degrees where

$\varphi = {\left( \frac{180}{\pi} \right){\theta.}}$

Throughout the remainder of this specification, phase angle will bepreferably referenced in terms of degrees. The three terminals comprise:(1) a first terminal designated “A-Catheter Tip” which is the tipelectrode 12; (2) a second terminal designated “B-Patch 1” such assource return patch electrode 22; and (3) a third terminal designated“C-Patch 2” such as the sense return patch electrode 20. In addition tothe ablation (power) signal generated by source 54 of ablation generator24, the excitation signal generated by source 56 in tissue sensingcircuit 26 is also be applied across the source connectors (SOURCE (+),SOURCE (−)) for the purpose of inducing a response signal with respectto the load that can be measured and which depends on the compleximpedance. As described above, in one embodiment, a 20 kHz, 100 μA ACconstant current signal is sourced along the path 60, as illustrated,from one connector (SOURCE (+), starting at node A) through the commonnode (node D) to a return patch electrode (, SOURCE (−), node B). Thecomplex impedance sensor 58 is coupled to the sense connectors (SENSE(+), SENSE (−)), and is configured to determine the impedance across thepath 62. For the constant current excitation signal of a linear circuit,the impedance will be proportional to the observed voltage developedacross SENSE (+)/SENSE (−), in accordance with Ohm's Law: Z=V/I. Becausevoltage sensing is nearly ideal, the current flows through the path 60only, so the current through path 62 (node D to node C) due to theexcitation signal is effectively zero. Accordingly, when measuring thevoltage along path 62, the only voltage observed will be where the twopaths intersect (i.e. from node A to node D). Depending on the degree ofseparation of the two patch electrodes (i.e., those forming nodes B andC), an ever-increasing focus will be placed on the tissue volume nearestthe tip electrode 12. If the patch electrodes are physically close toeach other, the circuit pathways between the catheter tip electrode 12and the patch electrodes will overlap significantly and impedancemeasured at the common node (i.e., node D) will reflect impedances notonly at the interface of the catheter electrode 12 and tissue 16, butalso other impedances between tissue 16 and the surface of body 17. Asthe patch electrodes are moved further part, the amount of overlap inthe circuit paths decreases and impedance measured at the common node isonly at or near the tip electrode 12 of catheter 14.

Referring now to FIG. 3, the concept illustrated in FIG. 2 is extended.FIG. 3 is a simplified schematic and block diagram of the three-terminalmeasurement arrangement of the invention. For clarity, it should bepointed out that the SOURCE (+) and SENSE (+) lines may be joined in thecatheter connector 40 or handle 42 (as in solid line) or may remainseparate all the way to the tip electrode (the SENSE (+) line beingshown in phantom line from the handle 42 to the tip electrode 12). FIG.3 shows in particular several sources of complex impedance variations,shown generally as blocks 64, that are considered “noise” because suchvariations do not reflect the physiologic changes in the tissue 16 orelectrical coupling whose complex impedance is being measured. Forreference, the tissue 16 whose complex impedance is being measured isthat near and around the tip electrode 12 and is enclosed generally by aphantom-line box 66 (and the tissue 16 is shown schematically, insimplified form, as a resistor/capacitor combination). One object of theinvention is to provide a measurement arrangement that is robust orimmune to variations that are not due to changes in or around box 66.For example, the variable complex impedance boxes 64 that are shown inseries with the various cable connections (e.g., in the SOURCE (+)connection, in the SOURCE (−) and SENSE (−) connections, etc.) mayinvolve resistive/inductive variations due to cable length changes,cable coiling and the like. The variable complex impedance boxes 64 thatare near the patch electrodes 20, 22, may be more resistive/capacitivein nature, and may be due to body perspiration and the like over thecourse of a study. As will be seen, the various arrangements of theinvention are relatively immune to the variations in blocks 64,exhibiting a high signal-to-noise (S/N) ratio as to the compleximpedance measurement for block 66.

Although the SOURCE (−) and SENSE (−) returns are illustrated in FIG. 3as patch electrodes 20, 22, it should be understood that otherconfigurations are possible. In particular, indifferent/dispersivereturn electrode 18 can be used as a return as well as another electrode50, 52 on catheter 14, such as ring electrode 50 as described incommonly assigned U.S. patent application Ser. No. 11/966,232 filed onDec. 28, 2007 and titled “SYSTEM AND METHOD FOR MEASUREMENT OF ANIMPEDANCE USING A CATHETER SUCH AS AN ABLATION CATHETER,” the entiredisclosure of which is incorporated herein by reference.

EP monitor 28 is provided display electrophysiology data including, forexample, an electrogram. Monitor 28 is conventional in the art and maycomprise an LCD or CRT monitor or another conventional monitor. Monitor28 may receive inputs from ablation generator 24 as well as otherconventional EP lab components not shown in the illustrated embodiment.

System 30 is provided for visualization, mapping and navigation ofinternal body structures. System 30 may comprise the system having themodel name EnSite NavX™ and commercially available from St. JudeMedical, Inc. and as generally shown with reference to commonly assignedU.S. Pat. No. 7,263,397 titled “Method and Apparatus for CatheterNavigation and Location and Mapping in the Heart,” the entire disclosureof which is incorporated herein by reference. System 30 may include anelectronic control unit (ECU) 32 and a display device 34 among othercomponents.

ECU 32 is provided to acquire values for first and second components ofa complex impedance between the catheter tip electrode 12 and tissue 16and to calculate a coupling index responsive to the values with thecoupling index indicative of a degree of coupling between electrode 12and tissue 16. ECU 32 may further calculate the coupling indexresponsive to a standardization value indicative of a deviation from areference standard by a parameter associated with at least one of thebody 17, the electrode 12 and another component of system 10. ECU 32preferably comprises a programmable microprocessor or microcontroller,but may alternatively comprise an application specific integratedcircuit (ASIC). ECU 32 may include a central processing unit (CPU) andan input/output (I/O) interface through which ECU 32 may receive aplurality of input signals including signals from sensor 58 of tissuesensing circuit 26 and generate a plurality of output signals includingthose used to control display device 34. In accordance with one aspectof the present invention, ECU 32 may be programmed with a computerprogram (i.e., software) encoded on a computer storage medium fordetermining a degree of coupling between an electrode on a catheter andtissue in a body. The program includes code for calculating a couplingindex responsive to values for first and second components of thecomplex impedance between the catheter electrode 12 and tissue 16 withthe coupling index indicative of a degree of coupling between thecatheter electrode 12 and the tissue 16. The program may further includecode for calculating the coupling index responsive to a standardizationvalue indicative of a deviation from a reference standard by a parameterassociated with at least one of body 17, electrode 12 and anothercomponent of system 10.

ECU 32 acquires one or more values for two component parts of thecomplex impedance from signals generated by sensor 58 of tissue sensingcircuit 26 (i.e., the resistance (R) and reactance (X) or the impedancemagnitude (|Z|) and phase angle (φ) or any combination of the foregoingor derivatives or functional equivalents thereof). In accordance withone aspect of the present invention, ECU 32 combines values for the twocomponents into a single coupling index that provides an improvedmeasure of the degree of coupling between electrode 12 and tissue 16and, in particular, the degree of electrical coupling between electrode12 and tissue 16.

Validation testing relating to the coupling index was performed in apre-clinical animal study. The calculated coupling index was compared topacing threshold as an approximation of the degree of coupling. Pacingthreshold was used for comparison because it is objective andparticularly sensitive to the degree of physical contact between the tipelectrode and tissue when the contact forces are low and the currentdensity paced into the myocardium varies. In a study of seven swine(n=7, 59+/−3 kg), a 4 mm tip irrigated RF ablation catheter wascontrolled by an experienced clinician who scored left and right atrialcontact at four levels (none, light, moderate and firm) based onclinician sense, electrogram signals, three-dimensional mapping, andfluoroscopic images. Several hundred pacing threshold data points wereobtained along with complex impedance data, electrogram amplitudes anddata relating to clinician sense regarding contact. A regressionanalysis was performed using software sold under the registeredtrademark “MINITAB” by Minitab, Inc. using the Log 10 of the pacingthreshold as the response and various impedance parameters as thepredictor. The following table summarizes the results of the analysis:

Regression R{circumflex over ( )}2 Model Regression Factors in ModelR{circumflex over ( )}2 R{circumflex over ( )}2_adj 1 R1_mean 43.60%43.50% (p < 0.001) 2 X1_mean 35.70% 35.50% (p < 0.001) 3 X1_mean R1_mean47.20% 46.90% (p < 0.001) (p < 0.001) 4 X1_stdev R1_stdev X1_meanR1_mean 48.70% 48.00% (p = 0.300) (p = 0.155) (p < 0.001) (p < 0.001) 5R1_P-P X1_stdev R1_stdev X1_mean R1_mean 49.00% 48.10% (p = 0.253) (p =0.280) (p = 0.503) (p < 0.001) (p < 0.001)

As shown in the table, it was determined that a mean value forresistance accounted for 43.5% of the variation in pacing thresholdwhile a mean value for reactance accounted for 35.5% of the variation inpacing threshold. Combining the mean resistance and mean reactancevalues increased the predictive power to 46.90% demonstrating that acoupling index based on both components of the complex impedance willyield improved assessment of coupling between the catheter electrode 12and tissue 16. As used herein, the “mean value” for the resistance orreactance may refer to the average of N samples of a discrete timesignal x_(i) or a low-pass filtered value of a continuous x(t) ordiscrete x(t_(i)) time signal. As shown in the table, adding morecomplex impedance parameters such as standard deviation and peak to peakmagnitudes can increase the predictive power of the coupling index. Asused herein, the “standard deviation” for the resistance or reactancemay refer to the standard deviation, or equivalently root mean square(rms) about the mean or average of N samples of a discrete time signalx_(i) or the square root of a low pass filtered value of a squared highpass filtered continuous x(t) or discrete x(t_(i)) time signal. The“peak to peak magnitude” for the resistance or reactance may refer tothe range of the values over the previous N samples of the discrete timesignal x_(i) or the k^(th) root of a continuous time signal[abs(x(t))]^(k) that has been low pass filtered for sufficiently largek>2. It was further determined that, while clinician sense alsoaccounted for significant variation in pacing threshold (48.7%)—and thusprovided a good measure for assessing coupling—the combination of thecoupling index with clinician sense further improved assessment ofcoupling (accounting for 56.8% of pacing threshold variation).

Because of the processing and resource requirements for more complexparameters such as standard deviation and peak to peak magnitude andbecause of the limited statistical improvement these parametersprovided, it was determined that the most computationally efficientcoupling index would be based on mean values of the resistance (R) andreactance (X). From the regression equation, the best prediction ofpacing threshold—and therefore coupling—was determined to be thefollowing equation (3):

ECI=Rmean−5.1*Xmean  (3)

where Rmean is the mean value of a plurality of resistance values andXmean is the mean value of a plurality of reactance values. It should beunderstood, however, that other values associated with the impedancecomponents such as a standard deviation of a component or peak to peakmagnitude of a component which reflect variation of impedance withcardiac motion or ventilation can also serve as useful factors in thecoupling index. Further, although the above equation and followingdiscussion focus on the rectangular coordinates of resistance (R) andreactance (X), it should be understood that the coupling index couldalso be based on values associated with the polar coordinates impedancemagnitude (|Z|) and phase angle (φ) or indeed any combination of theforegoing components of the complex impedance and derivatives orfunctional equivalents thereof. Finally, it should be understood thatcoefficients, offsets and values within the equation for the couplingindex may vary depending on among other things, the desired level orpredictability, the species being treated and disease states. Inaccordance with the present invention, however, the coupling index willalways be responsive to both components of the complex impedance inorder to arrive at an optimal assessment of coupling between thecatheter electrode 12 and tissue 16.

The above-described analysis was performed using a linear regressionmodel wherein the mean value, standard deviation and/or peak to peakmagnitude of components of the complex impedance were regressed againstpacing threshold values to enable determination of an optimal couplingindex. It should be understood, however, that other models and factorscould be used. For example, a nonlinear regression model may be used inaddition to, or as an alternative to, the linear regression model.Further, other independent measures of tissue coupling such as atrialelectrograms could be used in addition to, or as an alternative to,pacing thresholds.

Validation testing was also performed in a human trial featuring twelvepatients undergoing catheter ablation for atrial fibrillation. Thepatients were treated using an irrigated, 7 French radio frequency (RF)ablation catheter with a 4 mm tip electrode operating at a standardsetting of a 50° C. tip temperature, 40 W power and 30 ml/min. flow rate(adjusted accordingly proximate the esophagus). An experienced clinicianplaced the catheter in the left atrium in positions of unambiguousnon-contact and unambiguous contact (with varying levels of contactincluding “light,” “moderate,” and “firm”) determined throughfluoroscopic imaging, tactile feedback electrograms, clinicianexperience and other information. In addition to impedance, measurementsof electrogram amplitudes and pacing thresholds were obtained forcomparison. Each measure yielded corresponding changes in value as thecatheter electrode moved from a no-contact position to a contactposition. In particular, electrogram amplitudes increased from0.14+/−0.16 to 2.0+/−1.9 mV, pacing thresholds decreased from 13.9+/−3.1to 3.1+/−20 mA and the coupling index increased from 118+/−15 to145+/−24 (with resistance increasing from 94.7+/−11.0 to 109.3+/−15.1Ωand reactance decreasing from −4.6+/−0.9 to −6.9+/−2Ω). Further, thecoupling index increased (and resistance increased and reactancedecreased) as the catheter electrode was moved from a “no-contact”(115+/−12) position to “light,” (135+/−15) “moderate,” (144+/−17) and“firm” (159+/−34) positions. These measurements further validate the useof the coupling index to assess coupling between the catheter electrode12 and tissue 16. The calculated coupling index and clinician sense ofcoupling were again compared to pacing threshold as an approximation ofthe degree of coupling. A regression analysis was performed using alogarithm of the pacing threshold as the response and various impedanceparameters and clinician sense as predictors. From this analysis, it wasdetermined that clinician sense accounted for approximately 47% of thevariability in pacing threshold. The addition of the coupling index,however, with clinician sense resulted in accounting for approximately51% of the variability in pacing threshold—further demonstrating thatthe coupling index can assist clinicians in assessing coupling betweenthe catheter electrode 12 and tissue 16.

Referring now to FIGS. 4-5, a series of timing diagrams (in registrationwith each other) illustrate a comparison of atrial electrograms relativeto changes in resistance and reactance (FIG. 4) and the compositecoupling index (FIG. 5). As noted hereinabove, atrial electrograms areone traditional measurement for assessing coupling between the catheterelectrode 12 and tissue 16. As shown in FIG. 4, the signal amplitude ofthe atrial electrogram increases when the catheter electrode 12 movesfrom a position of “no contact” to “contact” with tissue 16. Similarly,measured resistance (R) increases and reactance (X) decreases and becomemore variable (FIG. 4) and the calculated coupling index increases (FIG.5), further demonstrating the utility of the coupling index in assessingcoupling between electrode 12 and tissue 16.

The human validation testing also revealed that the coupling indexvaried depending on tissue types. For example, the coupling index tendedto be higher when the catheter electrode was located inside a pulmonaryvein than in the left atrium. As a result, in accordance with anotheraspect of the present invention, the coupling index may be used inidentifying among tissue types (e.g., to identify vascular tissue asopposed to trabeculated and myocardial tissue). Further, because forcesensors may not adequately estimate the amount of energy delivered intotissue in constrained regions such as the pulmonary vein or trabeculae,the inventive coupling index may provide a more meaningful measure ofablation efficacy than force sensors. In addition, in certainsituations, it may be advantageous to utilize both a force sensor andthe coupling index.

Impedance measurements are also influenced by the design of catheter 14connection cables 56 or other factors. Therefore, the coupling index maypreferably comprise a flexible equation in which coefficients andoffsets are variable in response to design parameters associated withcatheter 14. Catheter 14 may include a memory such as an EEPROM thatstores numerical values for the coefficients and offsets or stores amemory address for accessing the numerical values in another memorylocation (either in the catheter EEPROM or in another memory). ECU 32may retrieve these values or addresses directly or indirectly from thememory and modify the coupling index accordingly.

The physical structure of the patient is another factor that mayinfluence impedance measurements and the coupling index. Therefore, ECU32 may also be configured to offset or normalize the coupling index(e.g., by adjusting coefficients or offsets within the index) responsiveto an initial measurement of impedance or another parameter in aparticular patient. In addition, it may be beneficial to obtain andaverage values for the coupling index responsive to excitation signalsgenerated by source 56 at multiple different frequencies.

In addition to the design of catheter 14 and the physical structure ofthe patient, variation in other components and/or characteristics ofsystem 10 can impact impedance measurements and the coupling index. Inaccordance with one aspect of the present teachings, therefore, ECU 32is configured to calculate the coupling index responsive to values forthe components of a complex impedance between electrode 12 and tissue 16and a standardization value indicative of a deviation from a referencestandard by a parameter associated with at least one of body 17,electrode 12 and another component of system 10. Exemplary parametersmay include the temperature, size, shape, surface area, body mass index,and electrolyte concentrations of body 17, the size of electrode 14, andthe size and location of electrodes 18, 20, 22.

The coupling index may be generally represented by the followingequation (4):

ECI=a*Rmean+b*Xmean+c  (4)

where Rmean is the mean value of a plurality of resistance values andXmean is the mean value of a plurality of reactance values and a, b, andc are coefficients acting as standardization values permittingstandardization of the coupling index by scaling or offset. Thecoefficients a, b, and c can be varied to address deviations fromreference standards by a wide variety of parameters. For parametersassociated with certain components of system 10, these deviations can beaddressed by establishing initial or default values for the coefficientsin the memory or code in ECU 32. In particular, impedance measurementsmay be impacted by circuit design (e.g., by employing amplifiers withdifferent gains or power supplies with different voltage limits) and bythe chosen format of values resulting from analog to digital conversion(e.g., the number of bits, signed vs. unsigned, integer vs. floatingpoint, etc.). Changes in impedance measurements resulting from changesin parameters associated with such components in system 10 can bequantified and used to adjust the coefficients in equation (4). Theequation (3) ECI=Rmean−5.1*Xmean recited hereinabove is one variation ofequation (4) in which the coefficient a equals 1, the coefficient bequals −5.1 and the coefficient c equals 0 as determined based ontesting one exemplary system 10.

For parameters relating to other components of system 10, deviationsfrom reference standards can be addressed by retrieving informationrelating to the component and adjusting the coefficients in equation (4)accordingly. As referenced hereinabove, changes in catheter 14 impactingimpedance measurements can be compensated for by adjusting thecoefficients using information retrieved from an EEPROM in catheter 14.It has been determined, for example, that that the size of electrode 12on catheter 14 impacts impedance measurements. In particular, smallerelectrodes (e.g. 2.5 mm) result in larger impedance measurements and inlarger changes in impendence when moving between contact and non-contactpositions relative to larger electrodes (e.g., 4 mm or 8 mm). An EEPROMin catheter 14 may include numerical values for the coefficients inequation (4) based on the size of the electrode (and/or other parametersassociated with catheter 14) or store a memory address for accessing thenumerical values in another memory location (either in the catheterEEPROM or in another memory). ECU 32 may retrieve these values oraddresses directly or indirectly from the memory and modify the couplingindex accordingly.

Variations in certain parameters from a reference standard may beaddressed dynamically during operation of system 10. Body temperatureand fluid conductivity in body 17 are two exemplary parameters. As bodytemperature increases, the conductivity of saline in catheter 14increases and the resistive component of impedance decreases. Bodytemperature may be measured using a conventional temperature sensor oncatheter 14. In response to the measured temperature, a standardizationvalue (e.g., one of coefficients a, b, and c in equation (4)) can beadjusted to compensate for a deviation in the measured temperature froma reference standard. The standardization value may be applied as anoffset (e.g., coefficient c in equation (4)) to the values of compleximpedance (e.g., Rmean and Xmean in equation (4)) or as a scaling of oneor both values (e.g., coefficient a or b in equation (4)). For example,if the reference standard body temperature is 37° C. and theconductivity of saline increases by 2% as temperature increases by eachdegree Celsius (and the resistive impedance decreases by 2% astemperature increases by each degree Celsius), ECU 32 may calculate thecoupling index ECI by adjusting the offset in equation (4) (assumingthat coefficients a, b, and c are previously optimized for a bodytemperature of 37° C.) as set forth in equation (5):

ECI=a*Rmean+b*Xmean+(c+0.02*a(T−37))  (5)

where T is the measured body temperature. Alternatively, ECU 32 maycalculate the coupling index ECI by adjusting the scaling of theresistive component of the complex impedance (because the temperaturedependence of impedance in saline is almost entirely resistive) inequation (4) as set forth in equation (6):

ECI=a(1+0.02(T−37))*Rmean+b*Xmean+c  (6)

where T again represents the measured body temperature.

For certain parameters that impact impedance—particularly thoseassociated with body 17, but also cabling and placement of electrodes18, 20, 22—variations relative to reference standards are not easilymeasured or controlled. Referring to FIG. 8, in accordance with oneaspect of the present teachings, a method for generating astandardization value is provided that accounts for variations in thesetypes of parameters. The method may begin with the step 84 of movingcatheter 14 relative to tissue 16 (e.g., within a cardiac chamber) untilcatheter 14 is in a non-contact position with tissue 16. The non-contactposition of catheter 14 relative to tissue 16 may be identified, forexample, based on a low value for the coupling index, electrogramamplitude or contact force or visually using fluoroscopy. The method maycontinue with the step 86 of recording a baseline or “non-contact” valueECI₀ for the coupling index while the catheter is in the “non-contact”position and the step 88 of retrieving a reference standard value of thecoupling index ECI*. The reference coupling index ECI* may be obtainedfrom a memory in ECU 32 and may represent an average or mean value forthe coupling index in a non-contact position across a predeterminedpopulation. Alternatively, the reference coupling index ECI* may beobtained by inserting catheter 14 into a calibration apparatus tosimulate a non-contact position. The method may continue with the step90 of generating a standardization value responsive to the baselinecoupling index ECI₀ and the reference coupling index ECI*—for example,based on the difference between the baseline coupling index ECI₀ and thereference coupling index ECI* (i.e. ECI*−ECI₀). The standardizationvalue may again be applied as an offset to the values for the componentsof complex impedance or as a scaling of one or both values.

Referring now to FIG. 6, display device 32 is provided to present thecoupling index in a format useful to the clinician. Device 32 may alsoprovide a variety of information relating to visualization, mapping andnavigation as is known in the art including measures of electricalsignals, two and three dimensional images of the tissue 16 andthree-dimensional reconstructions of the tissue 16. Device 32 maycomprise an LCD monitor or other conventional display device. Inaccordance with another aspect of the present invention, the couplingindex may be displayed in one or more ways designs to provide easyinterpretation and correlation to tissue contact for the clinician.Referring to FIG. 6, the coupling index may be displayed as a scrollingwaveform 68. The coupling index may also be displayed as a meter 70which displays the one second average value of the coupling index. Foreither the scrolling waveform 68 or meter 70, upper and lower thresholds72, 74 may be set (either pre-programmed in ECU 32 or input by the userusing a conventional I/O device). Characteristics of the waveform 68and/or meter 70 may change depending upon whether the value of thecoupling index is within the range set by the thresholds (e.g., thewaveform 68 or meter 70 may change colors such as from green to red ifthe value of the coupling index moves outside of the range defined bythe thresholds). Changes to the coupling index may also be reflected inchanges to the image of the catheter 14 and/or catheter electrode 12 ondisplay device 34. For example, the catheter electrode 12 may bedisplayed on the screen (including within a two or three dimensionalimage or reconstruction of the tissue) as a beacon 76. Depending on thevalue of the coupling index, the appearance of the beacon 76 may change.For example, the color of the beacon 76 may change (e.g., from green tored) and/or lines may radiate outwardly from the beacon 76 as the indexfalls above, below or within a range of values.

In summary, the degree of coupling between a catheter electrode 12 andtissue 16 may be assessed through several method steps in accordancewith one embodiment of the invention. First, an excitation signal isapplied between electrode 12 and a reference electrode such as patchelectrode 22 between connectors SOURCE (+) and SOURCE (−) along a firstpath 60 (see FIG. 2). As discussed above, signal source 56 of tissuesensing circuit 26 may generate the excitation signal at a predeterminedfrequency or frequencies. This action induces a voltage along path 62between electrode 12 and another reference electrode such as patchelectrode 20. The voltage may be measured by sensor 58 which resolvesthe sensed voltage into component parts of the complex impedance attissue 16. As a result, ECU 32 acquires values for the components of thecomplex impedance. ECU 32 then calculates a coupling index responsive tothe values that is indicative of a degree of coupling between theelectrode 12 and tissue 16. The index may then be presented to aclinician in a variety of forms including by display on display device34 as, for example, a waveform 68, meter 70 or beacon 76.

A coupling index formed in accordance with the teaching of the presentinvention may be useful in a variety of applications. As shown in theembodiment illustrated in FIG. 1, the coupling index can be used as partof a system 10 for ablation of tissue 16. The coupling index provides anindication of the degree of electrical coupling between tip electrode 12and tissue 16 thereby assisting in the safe and effective delivery ofablation energy to tissue 16.

The coupling index may further provide an indication of the proximity ororientation of the tip electrode 12 to adjacent tissue 16. Referring toFIGS. 1 and 2, signal source 56 of sensing circuit 26 may generateexcitation signals across source connectors SOURCE (+) and SOURCE (−)defined between tip electrode 12 and patch electrode 22 and also betweenring electrode 50 and patch electrode 22. Impedance sensor 58 may thenmeasure the resulting voltages across sense connectors SENSE (+) andSENSE (−)) defined between tip electrode 12 and patch electrode 20 andalso between ring electrode 50 and patch electrode 22. ECU 32 maycompare the measured values directly or, more preferably, determine acoupling index for each of electrodes 12, 50 responsive to the measuredvalues and compare the two indices. The comparison provides anindication of orientation of tip electrode 12. For example, a rise inthe measured impedance or coupling index for both electrodes 12, 50 mayindicate that electrode 12 is parallel to tissue 16. A rise in themeasured impedance or coupling index for electrode 12, but not forelectrode 50, may indicate that electrode 12 is perpendicular to tissue16. Differences between the measured impedance or coupling index forelectrodes 12, 50 may indicate that electrode 12 is disposed at an angle(as well as the degree of that angle) relative to tissue 16. It shouldbe understood that electrode 50 is used for exemplary purposes only.Similar results could be obtained with other electrodes disposedproximate tip electrode 12 or from using a split tip electrode.

The present invention may also be used as a proximity sensor. As anelectrode such as electrode 12 approaches tissue 16 the impedancechanges as does the coupling index. Further, for some electrodeconfigurations, this change is independent of the angle at which theelectrode is 12 is disposed relative to tissue 16. The coupling index istherefore indicative of the proximity of the electrode 12 to tissue 16.In some applications, the general position (with a frame of reference)and speed of the tip of catheter 14 and electrode 12 is known (althoughthe proximity of electrode 12 to tissue 16 is unknown). This informationcan be combined to define a value (the “coupling index rate”) that isindicative of the rate of change in the coupling index as electrode 12approaches tissue 16 and which may provide an improved measure of theproximity of the electrode 12 to tissue 16. This information can beused, for example, in robotic catheter applications to slow the rate ofapproach prior to contact and also in connection with a transseptalaccess sheath having a distal electrode to provide an indication thatthe sheath is approaching (and/or slipping away from) the septum. Thecoupling index rate can also be used to filter or smooth variation insignals resulting from cardiac cycle mechanical events.

The present invention may also find application in systems havingmultiple electrodes used for mapping the heart or other tissues,obtaining electrophysiological (EP) information about the heart or othertissues or ablating tissue. Referring to FIG. 7, one example of an EPcatheter 78 is shown. EP catheter 78 may be a non-contact mappingcatheter such as the catheter sold by St. Jude Medical, AtrialFibrillation Division, Inc. under the registered trademark “ENSITEARRAY.” Alternatively, catheter 78 may comprise a contact mappingcatheter in which measurements are taken through contact of theelectrodes with the tissue surface. Catheter 78 includes a plurality ofEP mapping electrodes 80. The electrodes 80 are placed within electricalfields created in body 17 (e.g., within the heart). The electrodes 80experience voltages that are dependent on the position of the electrodes80 relative to tissue 16. Voltage measurement comparisons made betweenelectrodes 80 can be used to determine the position of the electrodes 80relative to tissue 16. The electrodes 80 gather information regardingthe geometry of the tissue 16 as well as EP data. For example, voltagelevels on the tissue surface over time may be projected on an image orgeometry of the tissue as an activation map. The voltage levels may berepresented in various colors and the EP data may be animated to showthe passage of electromagnetic waves over the tissue surface.Information received from the electrodes 80 can also be used to displaythe location and orientation of the electrodes 80 and/or the tip of EPcatheter 78 relative to tissue 16. Electrodes 80 may be formed byremoving insulation from the distal end of a plurality of braided,insulated wires 82 that are deformed by expansion (e.g. through use of aballoon) into a stable and reproducible geometric shape to fill a space(e.g., a portion of a heart chamber) after introduction into the space.

In the case of contact mapping catheters, the coupling index can be usedto determine which electrodes 80 are in contact with or in closeproximity to tissue 16 so that only the most relevant information isused in mapping the tissue 16 or in deriving EP measurements or so thatdifferent data sets are more properly weighted in computations. As withthe systems described hereinabove, signal source 56 of sensing circuit26 may generate excitation signals across source connectors SOURCE (+)and SOURCE (−) defined between one or more electrodes 80 and patchelectrode 22. Impedance sensor 58 may then measure the resultingvoltages across sense connectors SENSE (+) and SENSE (−)) definedbetween each electrode 80 and patch electrode 20. ECU 32 may thendetermine which electrodes 80 have the highest impedance and/or couplingindex to determine the most relevant electrodes 80 for purposes ofmapping or EP measurements. Similarly, in the case of a multipleelectrode ablation catheter (not shown), the coupling index can be usedto determine which electrodes are in contact with tissue 16 so thatablation energy is generated through only those electrodes, or can beused to adjust the power delivered to different electrodes to providesufficient power to fully ablate the relevant tissue.

The present invention also permits simultaneous measurements by multipleelectrodes 80 on catheter 78. Signals having distinct frequencies ormultiplexed in time can be generated for each electrode 80. In oneconstructed embodiment, for example, signals with frequencies varying by200 Hz around a 20 kHz frequency were used to obtain simultaneousdistinct measurements from multiple electrodes 80. Because the distinctfrequencies permit differentiation of the signals from each electrode80, measurements can be taken for multiple electrodes 80 simultaneouslythereby significantly reducing the time required for mapping and/or EPmeasurement procedures. Microelectronics permits precise synthesis of anumber of frequencies and at precise quadrature phase offsets necessaryfor a compact implementation of current sources and sense signalprocessors. The extraction of information in this manner from aplurality of transmitted frequencies is well known in the field ofcommunications as quadrature demodulation. Alternatively, multiplemeasurements can be accomplished essentially simultaneously bymultiplexing across a number of electrodes with a single frequency forintervals of time less than necessary for a significant change to occur.

A system, article of manufacture, and method in accordance with thepresent teachings offers one or more of a number of advantages. First,they provide a true measure of impedance at the interface of theelectrode and the target tissue and, therefore, provide a morequalitative assessment of coupling between the electrode and the tissue.In particular, the system, article, and method exclude accumulatedimpedance between the target tissue and the body surface as well asimpedance resulting from external factors unrelated to the patient suchas cable length, coiling, and the like. The measurement is alsostandardized such that it will provide a similar measure despitevariation in parameters associated with the body, the electrode, and thesystem as a whole. Further, the system, article of manufacture andmethod provide an indicator of coupling (i.e., the coupling index) in aformat that allows easy interpretation and correlation to tissue contactby the clinician. As a result, ablation procedures can be conducted moreefficiently and with higher success rates and fewer complications.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the scope of this invention. All directional references (e.g.,upper, lower, upward, downward, left, right, leftward, rightward, top,bottom, above, below, vertical, horizontal, clockwise andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not as limiting. Changes in detail or structure may be made withoutdeparting from the invention as defined in the appended claims.

1-21. (canceled)
 22. A system for assessing a degree of coupling between a first electrode and a tissue in a body, comprising: a complex impedance sensor; and, an electronic control unit configured to acquire values for first and second components of a complex impedance between said first electrode and said tissue responsive to one or more signals output by said complex impedance sensor and to calculate a first coupling index responsive to said values and a first variable parameter associated with said system, said first coupling index indicative of a degree of coupling between said first electrode and said tissue.
 23. The system of claim 22 wherein said electronic control unit is further configured, in calculating said first coupling index, to apply an offset to said values of said first and second components of said complex impedance responsive to said first variable parameter.
 24. The system of claim 22 wherein said electronic control unit is further configured, in calculating said first coupling index, to scale at least one of said values of said first and second components of said complex impedance responsive to said first variable parameter.
 25. The system of claim 22 wherein said electronic control unit is further configured, in calculating said first coupling index to scale one of said values of said first and second components of said complex impedance responsive to said first variable parameter and scale another of said values of said first and second components of said complex impedance responsive to one of said first variable parameter and a second variable parameter associated with said system.
 26. The system of claim 22 wherein said electronic control unit is configured to calculate said first coupling index in accordance with the equation aR+bX+c wherein R and X correspond to said values for said first and second components of said complex impedance between said first electrode and said tissue and a, b, and c correspond to a first standardization value indicative of a deviation from a reference standard by said first variable parameter, a second standardization value indicative of a deviation from a reference standard by one of said first variable parameter and a second variable parameter associated with said system, and a third standardization value indicative of a deviation from a reference standard by one of said first variable parameter, said second variable parameter and a third variable parameter associated with said system.
 27. The system of claim 26 wherein R comprises a mean value of a plurality of values for said first component of said complex impedance and X comprises a mean value for a plurality of values for said second component of said complex impedance.
 28. The system of claim 22 wherein said first variable parameter comprises one or more of a temperature of said body, an electrolyte concentration in said body, and a size of said first electrode.
 29. The system of claim 22 wherein said electronic control unit is further configured to identify a type of said tissue responsive to said first coupling index.
 30. The system of claim 22 wherein said electronic control unit is further configured to determine at least one of a proximity of said first electrode to said tissue and an orientation of said first electrode relative to said tissue responsive to said first coupling index.
 31. The system of claim 22 wherein said electronic control unit is further configured to: acquire values for first and second components of a complex impedance between a second electrode and said tissue responsive to one or more signals output by said complex impedance sensor and to calculate a second coupling index responsive to said values for said first and second components of said complex impedance between said second electrode and said tissue and one of said first variable parameter and a second variable parameter associated with said system, said second coupling index indicative of a degree of coupling between said second electrode and said tissue; and, determine which of said first and second electrodes has a higher impedance responsive to said first and second coupling indeces.
 32. The system of claim 22, further comprising a display configured to display a representation of said first coupling index.
 33. The system of claim 32 wherein a characteristic of said representation assumes one of a first state and a second state depending on a value of said first coupling index relative to a threshold value.
 34. The system of claim 22, further comprising a display configured to display a representation of said first electrode wherein a characteristic of said representation assumes one of a first state and a second state depending on a value of said first coupling index relative to a threshold value.
 35. A method for assessing a degree of coupling between a first electrode and a tissue in a body, comprising: acquiring values for first and second components of a complex impedance between said first electrode and said tissue response to one or more signals output by a complex impedance sensor; and, calculating a first coupling index responsive to said values and a first variable parameter associated with a system controlling said first electrode, said first coupling index indicative of a degree of coupling between said first electrode and said tissue.
 36. The method of claim 35, wherein calculating said first coupling index includes applying an offset to said values of said first and second components of said complex impedance responsive to said first variable parameter.
 37. The method of claim 35, wherein calculating said first coupling index includes scaling at least one of said values of said first and second components of said complex impedance responsive to said first variable parameter.
 38. The method of claim 35, wherein calculating said first coupling index includes: scaling of one of said values of said first and second components of said complex impedance responsive to said first variable parameter; and scaling another of said values of said first and second components of said complex impedance responsive to one of said first variable parameter and a second variable parameter associated with said system.
 39. The method of claim 35 wherein said first coupling index in calculated in accordance with the equation aR+bX+c wherein R and X correspond to said values for said first and second components of said complex impedance between said first electrode and said tissue and a, b, and c correspond to a first standardization value indicative of a deviation from a reference standard by said first variable parameter, a second standardization value indicative of a deviation from a reference standard by one of said first variable parameter and a second variable parameter associated with said system, and a third standardization value indicative of a deviation from a reference standard by one of said first variable parameter, said second variable parameter and a third variable parameter associated with said system.
 40. The method of claim 39 wherein R comprises a mean value of a plurality of values for said first component of said complex impedance and X comprises a mean value for a plurality of values for said second component of said complex impedance.
 41. The method of claim 35 wherein said first variable parameter comprises one or more of a temperature of said body, an electrolyte concentration in said body, and a size of said first electrode.
 42. The method of claim 35, further comprising one or more of identifying a type of said tissue, determining a proximity of said first electrode to said tissue, and determining an orientation of said first electrode relative to said tissue responsive to said first coupling index.
 43. The method of claim 35, further comprising: acquiring values for first and second components of a complex impedance between a second electrode and said tissue responsive to one or more signals output by said complex impedance sensor; calculating a second coupling index responsive to said values for said first and second components of said complex impedance between said second electrode and said tissue and one of said first variable parameter and a second variable parameter associated with said system, said second coupling index indicative of a degree of coupling between said second electrode and said tissue; and, determining which of said first and second electrodes has a higher impedance responsive to said first and second coupling indeces.
 44. The method of claim 35, further comprising displaying a representation of said first coupling index wherein a characteristic of said representation assumes one of a first state and a second state depending on a value of said first coupling index relative to a threshold value.
 45. The method of claim 35, further comprising displaying a representation of said first electrode wherein a characteristic of said representation of said first electrode assumes one of a first state and a second state depending on a value of said first coupling index relative to a threshold value. 